Significance This work is undertaken to advance our knowledge of ocular mechanics and our treatment of strabismus. It will lead to a better understanding of muscle paths in normal and strabismic eyes, and of the effects of strabismus surgery on muscle paths. This will allow us to create more realistic computer models, which will facilitate better diagnoses, customized treatments, and the development of new surgeries and other treatments. Developing the monkey orbit as a model for the human orbit will facilitate the use of this animal model in strabismus. We will better understand the response of EOMs to surgical stretching and relaxation, and the possible involvement of proprioception in strabismus. Aims and Methods (1) We will clarify the functional anatomy of strabismic and normal orbits, testing the hypothesis that musculo-orbital coupling is mechanically significant: (1.1) Using magnetic resonance imaging with a surface coil, we will visualize muscle paths as a function of gaze in strabismic humans before and after surgery; (1.2) Using a scleral contact ring to apply torsion to normal eyes during MRI scanning, we will clarify the mechanics of orbital fascial interconnections, as they affect muscle paths. (2) Using a monkey trained to fixate, we will measure the innervation-length-tension relationships of horizontal rectus muscles. (3) We will study slow changes in EOM force following muscle resection surgery. Our method will determine if such changes occur and, if so, discriminate changes in elasticity from changes in innervation due to proprioceptive feedback. (4) We will continue development and testing of the SQUINT computer strabismus model with clinical data.